Coat-stripping unit and method for its operation

ABSTRACT

A coupling ( 7 ) which is rotatable about a vertical axis of rotation and intended for the mounting of a workpiece support ( 15 ) which can be loaded, for example, with incorrectly coated or worn workpieces ( 18 ) is arranged in a removable holder ( 5 ) above a tank ( 2 ) filled with an electrolyte. Said workpiece support is connected via the coupling ( 7 ) to a current supply device, to the opposite pole of which an opposite electrode ( 12 ) is connected. A heating and cooling device ( 13 ) and an ultrasound generator ( 14 ) are also arranged in the tank ( 2 ). By current supply and simultaneous rotation, the coat is stripped from the workpiece support ( 15 ) and/or from the workpieces ( 18 ). After incorrect coating, the workpieces ( 18 ) together with the workpiece support ( 15 ) and in certain circumstances with the holder ( 5 ) can be taken over directly from a coating unit and, after coat stripping, transferred to such a coating unit for recoating.

FIELD OF THE INVENTION

The invention relates to a coat-stripping unit and a method for its operation. Coat-stripping units are used for freeing workpiece supports which are used in coating units and are inevitably also coated there from their coating and for making them suitable for use again and also for stripping the coat from worn or incorrectly coated workpieces and thus preparing them for recoating.

THE PRIOR ART

Mechanical methods, especially sand blasting, are predominantly used today for stripping the coat from workpiece supports. The workpiece supports have to be dismantled for this purpose. After the treatment, the sand has to be carefully removed, for example by blowing away. Nevertheless, frequently residues cannot be avoided, which may subsequently lead to malfunctions. Moreover, the workpiece supports are unevenly worn during the sand blasting and are subject to considerable wear, which shortens their life.

For stripping the coat from worn and incorrectly coated workpieces, electrochemical methods are also used, cf. for example, WO 99/54 528 A1. Coat-stripping units according to the precharacterizing clause of claim 1 are used for this purpose. Some of these units have baskets or rotating drums which can be moved up and down for receiving the workpieces. In general, however, the workpieces have to be individually contacted. This is particularly complicated in the removal of layers of non-conductive material where the contacting is subject to further limitations and the workpieces have to be mounted individually on holders.

The formation and positioning of the opposite electrode surface in known coat-stripping units of the generic type are difficult and cannot always be optimally realised, since the current flow should be distributed as uniformly as possible over those surface regions of the workpieces from which the coat is to be stripped, so that uniform coat stripping is achieved and corrosion of the body bearing the coating is avoided.

SUMMARY OF THE INVENTION

It is the object of the invention further to develop coat-stripping units of the generic type so that the process of coat stripping is substantially simplified and can be carried out with great reliability. This object is achieved by the features in the characterizing clause of claim 1. In addition, methods for operating a coat-stripping unit according to the invention are described, by means of which coat stripping and also, in addition to the coat stripping, more complex processes comprising further steps can be carried out in a simple manner.

Advantages of the invention are in particular that stripping the coat from workpiece supports can be effected substantially more easily and more gently. Since mechanical processing can be substantially avoided, the life is substantially longer. After treatment is necessary at most to a small extent (washing, spraying, drying) and requires scarcely any manual work. In addition, however, stripping the coat from worn and incorrectly coated workpieces is also substantially simplified and can be effected far more reliably. By turning the workpiece support, a more or less uniform current density—averaged over the processing time—can be achieved more easily. Particularly in the case of workpieces having a non-conductive coating, the handling is generally substantially simplified since the workpieces are contacted in exactly the same way as in the case of the coating during the coat stripping.

Particularly if incorrectly coated workpieces are to be stripped of the coat and recoated, the handling effort can be substantially reduced if the workpieces are processed by means according to the invention. For coat stripping, they remain in this case on the workpiece support on which they were originally coated. Removal and remounting are dispensed with since the workpiece support with the mounted and contacted workpieces is transferred from a coating unit to the coat-stripping unit according to the invention and inserted therein. In the case of recoating of the workpieces, the workpiece support with the workpieces is transferred in the same way after coat stripping to a coating unit and inserted therein, whereupon the workpieces are recoated. Direct handling of the workpieces can be dispensed with.

BRIEF DESCRIPTION OF THE DRAWINGS

Below, the invention is explained in more detail with reference to a figure which shows only an embodiment.

FIG. 1 shows a vertical section through a coat-stripping unit according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The coat-stripping unit according to the invention comprises a cuboid housing 1 which contains a tank 2 which consists of non-conductive material or is coated on the inside with a non-conductive material so that the inner surface is non-conductive. It serves for holding a liquid electrolyte. An overflow 3 with a screen or a filter is arranged next to the tank 2. At the top, the housing 1 has an opening 4.

A holder 5 comprises a baseplate 6 by means of which it is supported on the housing 1 and a coupling 7 which is rotatably mounted in the baseplate 6. Its axis of rotation runs vertically and approximately centrally through the tank 2. A drive device 8 which is in the form of an electric motor has an operative connection to the coupling 7. The holder 5 likewise comprises a two-pole current supply device (not shown), one pole of which has an electrically conductive connection via a current transmission device 9 to the coupling 7. It may be in the form of a current source, a voltage source, pulsed current source or pulsed voltage source or in the form of an alternating current supply. The holder 5 is connected to the housing 1 so that it can be removed without great effort, for example simply lifted off. The current supply device may also be external so that the coat-stripping unit has only connections for connection to said current supply device, one connection of which is connected to the current transmission device 9. Moreover, a frame surrounding the axis of rotation is provided with a plurality of parallel rods 10 which are anchored to the baseplate 6 and on the ends of which a cross-strut 11 is held.

The other pole of the current supply device or the other connection is connected to an opposite electrode 12 which, for example, is in the form of a grid electrode and is arranged in the interior of the tank 2. The opposite electrode 12 has an opposite electrode surface facing the axis of rotation, approximately equidistant from said axis everywhere and extending substantially over the entire height of the tank 2. Said opposite electrode surface may have, for example, the form of a concave cylinder lateral surface, the axis of which coincides with the axis of rotation and which extends over a smaller or larger sector of, for example, 90° to 180°. However, the opposite electrode may also be in the form of a rod electrode. A heating and cooling device 13 and an ultrasound generator 14 and an inlet for the electrolyte and devices for moving said electrolyte, such as pumps or stirrers (not shown), are also arranged in the tank 2.

During operation, the tank 2 is filled up to at least close to the upper edge with a liquid electrolyte. A workpiece support 15 which is in the form of a rotation-symmetric multilevel support with a central support shaft (in the following called support tree) in the example, in particular a central shaft 16 of said workpiece support which lies in the axis of rotation, is non-rotatably connected to the coupling 7, to which shaft discs 17 are fastened in succession at regular intervals, which discs carry workpieces 18 distributed in each case over their periphery. There are various possibilities for the formation of the connection between the coupling 7 and the shaft 16. Rapidly producible and detachable connections, such as plug connections or bayonet connections, are preferred. The tip of the shaft 16 is rotatably mounted on the cross-strut 11. The shaft 16 is connected to the first pole of the current supply device via the coupling 7 and the discs 17 too are connected via the shaft. The workpieces 18 are fastened to the discs 17 and contacted so that they too have an electrically conductive connection to the first pole of the current supply device. The workpiece support 15 as well as all other components of the coat-stripping unit which are arranged in the tank 2 must consist of a material which is not attacked by the electrolyte, usually of stainless steel or, where this is not possible, as in the case of some magnetic materials, must be encapsulated in stainless foil.

The workpiece support may also be formed other than as described; in principle, it is possible to use any type of workpiece support which is rotatable about an axis of rotation and is used in coating units, in particular vacuum coating units, for example support trees with multiple rotation, it being possible for driver fingers or workpiece supports of another type having magnetic or plug plates to be mounted, for example, on the rods 10. The fastening of the workpieces can be effected in the manner customary in vacuum coating units, for example by plug or clamp connections or magnetically.

The current supply device now produces an intermittent or permanent potential difference between the workpiece support 15 acting as electrode and the workpieces 18 contacted via said workpiece support on the one hand and the opposite electrode 12 on the other hand. Where applicable, the electrode is usually connected as the anode and the opposite electrode is the cathode. At the same time, the coupling 7 is rotated uniformly by the drive device 8, and with it the workpiece support 15, the rotational speed being adapted to the process. The workpieces 18 are moved closely past the opposite electrode surface, preferably at a minimum distance of between 2 cm and 20 cm, preferably between 3 cm and 8 cm. The coat stripping is now effected as a rule by electrochemical dissolution of the coating of both the workpiece support 15 and the workpieces 18. Owing to the rotation of the workpiece support 15, the current density is fairly uniform on average as a function of time, i.e. the current is uniformly distributed over the surfaces from which the coat is to be stripped and the coat stripping is accordingly effected uniformly and in a gentle manner. This is also assisted by the fact that, in any case according to a preferred development, all parts of the opposite electrode surface are about the same distance away from the axis of rotation and hence from the shaft 16.

It is also possible for the coating to consist of an adhesive layer deposited directly on the body and a functional layer deposited on said adhesive layer and for the coat stripping to be effected by dissolution of the adhesive layer through pores of the functional layer, as described, for example in WO 99/64 646 A1 and WO 05/073 433 A1. Especially in such cases, the coat-stripping process can be substantially accelerated and improved by the use of the ultrasound generator 14.

Various modes of operation are possible with the coat-stripping unit according to the invention. Thus, it can be used, for example, for stripping the coat from a workpiece support 15 alone, i.e. a workpiece support without workpieces. Alternatively it can be used for stripping the coat from workpieces incorrectly coated beforehand in a coating unit, in particular a vacuum coating unit. In this case, for example, the holder 5 together with the workpiece support 15 and workpieces 18 can be removed from the coating unit and inserted into the coat-stripping unit without it being necessary to carry out any other manipulations on these parts.

The same procedure can be adopted in the case of recoating after coat stripping is complete. In this case, the holder together with the workpiece support 15 and the workpieces 18 can be removed from the housing 1 of the coat-stripping unit and transferred to a coating unit, in particular a vacuum coating unit, and inserted into the vacuum chamber thereof and, for example, coated by the CVD or PVD process.

In a multi-chamber unit, it is also possible, between coat stripping and recoating, to carry out other steps in suitable chambers, such as washing, spraying and drying, all without having to separate the workpiece support 15 from the holder 5 or even having to remove the workpieces 18. Of course, coat stripping and recoating can also be carried out in this manner in succession after incorrect coating. A suitable transport device can be provided for moving the holder 5 between the chambers. In this way, even complex processes comprising a plurality of processing steps can be substantially automated.

Alternatively, it is possible to move in each case the workpiece support 15 alone, equipped with workpieces 18, in the manner described, it being necessary to mount said workpiece support on the coupling 7 and corresponding couplings in the other chambers or to uncouple it from them. This is somewhat more complex but the requirements which the transport device has to meet may be lower.

Many modifications of the coat-stripping unit described are also possible. Thus, a plurality of couplings for a plurality of workpiece supports can be provided in the tank. Instead of being arranged above the tank or in its upper region, the coupling can also be arranged on its bottom. The tank can be formed in such a way that other process steps, such as washing, spraying and drying, can also be carried out in it. For this purpose, pumps and a collecting volume for temporary removal of the electrolyte can be provided.

Some examples for possible developments of coat-stripping units according to the invention and methods in which they are used are given below:

1. Holders for plungers of piston valves which became coated with a multi-layer coat up to 100 μm thick after repeated use in coating processes for depositing DLC and CrN/DLC coats were loaded, on support trees, on a plurality of rotatable couplings mounted in the bottom of a tank. The individual support trees were each fastened to a coupling electrically conductive in the contact region, the drivers were positioned against the satellites in a manner similar to that in PVD units and the tank was then filled with a potassium hydroxide-containing electrolyte to which phosphates and surfactants had been added and which had a pH of 11 to 13. With constant revolution, the tree with double-rotating satellites was stripped at room temperature and a constant +12 V voltage in the course of 1 h. The electrolyte attack took place in each case through the pores of the DLC layer to the CrN layer or the thin Cr layer deposited under the DLC as an adhesive layer.

2. Trees with double-rotating satellites for bucket tappets which became coated after repeated use with DLC and CrN/DLC layers were stripped similarly to example 1 with use of the same electrolyte at a constant 200 A current at room temperature in the course of 1 h. Instead of being driven by driver springs, the double rotation was effected here by means of toothed wheels and toothed rings.

3. Incorrectly coated plungers of piston valves which were coated with a DLC layer, a WC/C layer deposited on a Cr adhesive layer or CrN/DLC double layer were loaded onto satellite discs and trees as for a PVD coating process. Depending on the size of the control pistons, discs having different divisions were used. The trees were fastened in succession in a one-tree device on a coupling mounted in the bottom of a tank and, after positioning of driver springs, were immersed in the alkaline electrolyte of example 1. With constant double satellite rotation and constant 250 A current, the trees were stripped at room temperature in the course of 10 min.

4. In the device used in example 3, CrN+DLC-coated bolts and piston pins loaded onto satellite trees were stripped in the same electrolyte as in example 1 at constant 15 V in 5 min.

5. HSS drills and cutters and other HSS tools which were coated with different chromium-containing coat systems, such as Balinit Alcrona (aluminium chromium nitride), Balinit Helica (aluminium chromium silicon nitride), Balinit C (Cr+WC/C), Balinit DLC (Cr+DLC) and Balinit D (CrN) were loaded together onto satellite discs and installed analogously to example 3 on trees in the tank. After immersion in the alkaline electrolyte of example 1, the tools were stripped at room temperature with constant double rotation at a current of 250 A in the course of 30 min.

6. A similar mixture of differently coated tools was installed as in example 5 and then stripped in one percent sodium hydroxide solution at a constant 12 V voltage under otherwise identical conditions in 25 min.

7. Two HSS hobs coated with Balinit Alcrona were loaded on to a tree and the latter was immersed in the electrolyte of example 6. With constant single rotation, they were stripped at a constant 12 V voltage in the course of 20 min.

8. A disc with drills and cutters which were coated with Balinit Futura-Nano (TiAlN) and Balinit Alcrona (AlCrN) were loaded on to a tree and inserted upside down into a device according to FIG. 1 and immersed in the electrolyte of example 1. With constant single rotation, the surface was modified by formation of a thin layer of an aluminium oxide with an oxonitride-containing transition zone to the TiAlN layer at a constant 6 V voltage in the course of 3 min.

9. Carbide tools which were coated with different coats, such as Balinit Futura (titanium aluminium nitride), Balinit Xceed (TiAln having a high aluminium content), Balinit AlCrona (AlCrN) and Balinit Helica (aluminium chromium silicon nitride) were loaded together onto satellite discs and immersed analogously to example 8 in on trees in an electrolyte comprising ammonium nitrate and acetic acid with a pH of 1.5 to 3. With constant single rotation, the tools were stripped at a voltage of 15 V in the course of 5 min.

10. Trees with coated bolts which, as a result of the process, were coated with a defective double layer consisting of CrN+WC/C were removed directly from the PVD coating unit and stripped in a device analogously to example 3 and under the same conditions as in example 3. Thereafter, the trees with the bolts were spray-cleaned several times in the tank and dried with warm air. The trees were then installed directly in the PVD coating unit for recoating.

In addition to being used for coat stripping, the unit according to the invention can also be used for other electrochemical processes, for example for modifying coatings as in example 8 or for polishing or oxidizing surfaces of workpieces, defined rounding of edges on cutting tools, etc. Such processes, too, are made uniform through the rotation of the workpiece support. In addition, they can be better coordinated with other steps of a complex process.

List of Reference Numerals

-   1 Housing -   2 Tank -   3 Overflow -   4 Opening -   5 Holder -   6 Baseplate -   7 Coupling -   8 Drive device -   9 Current transmission device -   10 Rods -   11 Cross-strut -   12 Opposite electrode -   13 Heating and cooling device -   14 Ultrasound Generator -   15 Workpiece support -   16 Shaft -   17 Disc -   18 Workpieces 

1. Coat-stripping unit comprising a tank for holding a liquid electrolyte and an opposite electrode surface arranged in the interior of the tank and capable of being connected to a pole of a current supply device, wherein it comprises a holder having a coupling rotatably mounted on an axis of rotation which passes through the interior of the tank and intended for mounting of a workpiece support, which coupling can be connected to a further pole of the current supply device to permit the connection of such a workpiece support, and having a drive device operatively connected to the coupling and intended for rotation thereof.
 2. Coat-stripping unit according to claim 1, wherein the axis of rotation is vertical.
 3. Coat-stripping unit according to claim 1, wherein the coupling is arranged above the tank or in the upper part thereof.
 4. Coat-stripping unit according to claim 3, wherein the holder can be removed from the tank.
 5. Coat-stripping unit according to claim 1, wherein the opposite electrode surface faces the axis of rotation and its distance from the axis of rotation along said axis of rotation is substantially constant.
 6. Coat-stripping unit according to claim 5, wherein all parts of the opposite electrode surface are approximately the same distance away from the axis of rotation.
 7. Coat-stripping unit according to claim 1, wherein the opposite electrode surface is part of the surface of at least one opposite electrode arranged in the interior of the tank and a distance away from the inner surface thereof.
 8. Coat-stripping unit according to claim 7, wherein the inner wall of the tank consists of electrically insulating material.
 9. Coat-stripping unit according to claim 1, wherein an ultrasound generator is arranged in the interior of the tank.
 10. Coat-stripping unit according to claim 1, wherein a heating and cooling device is arranged in the interior of the tank.
 11. Method for operating a coat-stripping unit according to claim 1, wherein a workpiece support at least predominantly consisting of electrically conductive material is mounted on the coupling, the interior of the tank is filled with a liquid electrolyte and, for coat stripping, different electrode potentials are applied at least intermittently by means of the current supply device to the coupling and via this to the workpiece support as an electrode on the one hand and to the opposite electrode surface on the other hand, while the workpiece support, substantially completely immersed in the electrolyte, is rotated by rotating the coupling by means of the drive device.
 12. Method according to claim 11, wherein the workpiece support is a workpiece support used before in a coating unit.
 13. Method according to claim 11 for stripping the coat from workpieces, wherein the workpiece support is loaded with the workpieces from which the coat is to be stripped.
 14. Method according to claim 13, for stripping the coat from incorrectly coated workpieces, wherein, before the coat stripping, the workpiece support loaded with the incorrectly coated workpieces is removed from a coating unit and inserted into the coat-stripping unit.
 15. Method according to claim 13, for stripping the coat from workpieces and recoating said workpieces, wherein, after the coat stripping, the workpiece support, loaded with the workpieces, is removed from the coat-stripping unit and inserted into a coating unit and the workpieces are then recoated therein.
 16. Method according to claim 15, wherein the workpiece support, loaded with the workpieces, is subjected to at least one treatment, e.g. washed, sprayed, dried, between the coat stripping and the transfer to the coating unit. 